Support async method definitions in the py domain

[brettcannon]

Python 3.5 adds async methods. It would be great to has a directive like .. async:: to define a method as asynchronous. Not sure how to render it since def isn't rendered for .. method::, but maybe await can be used to denote the asynchronous method since await is appropriate for the method call?

[Rapptz]

Libraries such as aiohttp and I believe even the Python stdlib themselves use something akin to this. I suppose it should be fairly easy to add something like this to the main Sphinx py domain.

The actual Python stdlib has other useful ones for decorators and the such.

[tk0miya]

Done in #6295.
Thanks,